The identification and modelling of rockfalls for protection measures in the Western Cape

Surujbally, Shivana (2016-03)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Natural slopes and man-made cuttings in weathered, highly jointed or fractured rock with unfavourable bedding planes can result in the release of rock blocks either from above these slopes and cuttings or from the face itself resulting in what is termed a “Rockfall”. Rockfalls are a global phenomenon and much research has been done in countries like Switzerland, Italy, United States of America and Canada. Some of these countries have contributed to the field by developing their own software packages to model potential rockfalls or those rockfalls that have already occurred and assist in locating the position of rockfall mitigation measures such as catch fences. This research project covers the occurrence of rockfalls at three specific sites within the Western Cape in South Africa. This project was undertaken as very little is known about rockfalls in the Western Cape, let alone South Africa. And what little is known is not well documented or published. Three sites were chosen which included Chapman’s Peak Drive, Sir Lowry’s Pass and The Cliffs. Chapman’s Peak Drive is a world renowned pass located on the Western side of the peninsula, whilst Sir Lowry’s Pass is located between Somerset West and Grabouw and forms part of the National Route 2. The Cliffs used to serve as a hard rock quarry and is located in Tygervalley and now forms the backdrop to office complexes. The geology of two of the three sites that were studied is located in the Malmesbury Group, which covers a large percentage of the Western Cape, whereas the third site is located in both the Table Mountain Supergroup and the Cape Granite. The aim of the research project was to combine the collection of data through fieldwork with rockfall software programs in areas that are known to have experienced rockfalls to assist in mitigating the problem by avoiding over design and thereby reducing the overall cost of the project. This was achieved by the use of ArcGIS 10.0, Rapid Mass Movement System (RAMMS) Rockfall (trial version) and Rocscience Rocfall 4.0. Field inspections were carried out for each of the study areas, followed by terrestrial surveys. The terrestrial survey information was used to produce an ASCII file which is the required input file for RAMMS and produces 3D trajectory paths when a rockfall simulation is run. The trajectories were exported from RAMMS and draped over a 3D rendered a view of each site Stellenbosch University https://scholar.sun.ac.za iv for better viewing of the paths. These trajectories were then traced in AutoCAD and imported into the 2D Rocscience Rocfall software. The rockfall simulation was carried out in Rocfall and are presented in this research project. The outcome of this project concluded that rockfall software programs are undoubtedly useful in engineering design to mitigate rockfalls and reduce overall costs of a project. However, like with anything else man-made, software programs are not without flaws and cannot replace an experienced person in the field. Unrealistic software results can only be distinguished by someone who understands the behaviour of rockfalls as certain parameters such as the coefficients of restitution require educated assumptions since there is no South African field tested database currently. Although creating a local database of these parameters will be a costly undertaking, it would significantly improve the confidence for results produced by the rockfall software programs and therefore, this warrants further research into this field.

AFRIKAANSE OPSOMMING: Natuurlike hellings en mensgemaakte deurgrawings in verweerde, genate of gebreekte gesteentes met ongunstige laagvlakke, kan daartoe lei dat gesteenteblokke van hoër areas of van die rotswand self, los kom. Hierdie verskynsel staan bekend as ‘n “rotsstorting”. Rotsstortings is globale verskynsels en baie navorsing is uitgevoer in lande soos Switserland, Italië, die Verenigde State van Amerika en Kanada. Van hierdie lande het bygedra tot die veld deur hul eie sagteware pakkette te ontwikkel wat potentiële rotsstortings of rotsstortings wat reeds plaasgevind het, te modelleer en om die posisie van rotsstorting versagtingmaatreëls, soos vangnette, vas te stel. Hierdie navorsingsprojek handel oor rotsstortings by drie spesifieke terreine in die Wes Kaap in Suid Afrika. Die projek was uitgevoer omrede daar baie min inligting beskikbaar is oor rotsstortings in die Wes Kaap en, nog meer, Suid Afrika. Die inligting wat bekend is, is nie goed gedokumenteer of gepubliseer nie. Drie terreine was gekies vir die studie en sluit in Chapman’s Peak Rylaan, Sir Lowry’s Pas en The Cliffs. Chapman’s Peak Rylaan is ‘n wêreldbekende pas wat geleë is aan die Westekant van die skiereiland, terwyl Sir Lowry’s Pas tussen Somerset-Wes en Grabouw geleë is. Die laasgenoemde pas vorm deel van die N2 (Nationale Roete 2). The Cliffs was in die verlede benut as ‘n steengroef en is geleë in Tygervallei. Dit vorm tans die agtergrond vir kantoor komplekse. Twee van die drie terreine wat bestudeer is, is geleë in die Malmesbury Groep wat ook ‘n groot deel van Wes-Kaap gesteentes uitmaak. Die derde terrein is geleë in beide die Tafelberg Supergroep en Kaapse Graniet. Die doel van die navorsingsprojek was om die versamelde gebiedsinligting te kombineer met rotsstorting sagteware pakette vir areas waar rotsstortings al plaasgevind het. Hierdeur kan die probleem versag word deur oor-ontwerp te voorkom en daardeur die algehele koste van die projek te verminder. Dit was bereik deur die gebruik van ArcGIS 10.0, Rapid Mass Movement System (RAMMS) Rockfall (proef weergawe) en Rocscience Rocfall 4.0. Gebied Stellenbosch University https://scholar.sun.ac.za vi inspeksies was uitgevoer vir elk van die studie areas, gevolg deur landelike opnames. Die informasie verkryg deur die landelike opnames was gebruik om ‘n ASCII lêer te maak, wat die benodigde invoer lêer vir RAMMS is. Wanneer ‘n rotsstorting simulasie gehardloop word, word ‘n 3D trajek geproduseer. Die trajekte was uitgevoer vanaf RAMMS en gedrapeer oor ‘n 3D beeld van elk van die drie terreine vir ‘n verbeterde aansig van die trajekte. Hierdie trajekte was oorgeteken in AutoCAD en in die 2D Rocsceience Rocfall sagteware ingevoer. Die simulasie van die rotsstorting was uitgevoer in Rocfall en word voorgelê in hierdie navorsingsprojek. Daar kan tot die gevolgtrekking gekom word dat rotsstorting sagteware programme ongetwyfeld nuttig in die ontwerp van rotsstorting versagtingsmaatreëls is en daarby die algehele koste van die projek verlaag. Hierdie sagteware programme is, soos enige mensgemaakte verkynsel, nie sonder foute nie en kan ‘n ervare persoon op terrein nie vervang nie. Onrealistiese resultate van sagteware kan slegs uitgeken word deur ‘n persoon wat die gedrag van rotsstortings verstaan aangesien sekere parameters soos die koëffisiënt van restitusie ‘n ingeligte skatting vereis omdat daar tans geen databasis met gebiedsinligting beskikbaar is in Suid-Afrika nie. Alhoewel die maak van ‘n plaaslike databasis met hierdie parameters ‘n duur proses sal wees, sal dit die selfvertroue rondom resultate geproduseer deur rotsstorting sagteware programme aansienlik verbeter, en om hierdie rede kan verdere navorsing in hierdie gebied geregverdig word. Stellenbosch University https://scholar.sun.ac.za

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